Computer keeps turning itself on

[Lysander]

For some reason my 2nd computer [running XP SP2] seems to like turning itself on for no reason.

This generally happens after I shut down. The PC will either restart instantly instead of shutting down or it'll happen after about 30 seconds.

So I put in a new hard drive and reinstalled XP and it's still happening! Yesterday I shut down and stared at it for about a minute while it was off thinking "right, it's definitely shut down now". I left the house, came back a few hours later and it was on!!

Either we've got ghosts or something else is wrong with it. Anyone know what?

 

[Lysander]

It sounds to me like a PSU problem this, since it definately isn't software related and also the fact that when you switch off it powers back up on its own accord. How old is this PSU, what is it? and can you test it on another system?

Well, I changed the PSU in December and it happened before that too, so it can't be the PSU.